cleanup: remove dead code + deduplicate GPU state reads

Dead code removed:
- globals.py: GUILD_SETTINGS (empty dict, zero consumers)
- config.py: unused 'import os'
- config_manager.py: unused 'import os' and 'Union'
- server_manager.py: duplicate 'from datetime import datetime, timedelta'

GPU deduplication:
- get_current_gpu_url() now delegates to config_manager.get_gpu()
- get_gpu_status() endpoint now delegates to config_manager.get_gpu()
- Both previously re-read memory/gpu_state.json directly

bot/config.py

@@ -5,7 +5,6 @@ Uses Pydantic for type-safe configuration loading from:
 - config.yaml (all other configuration)
 """
 
-import os
 from pathlib import Path
 from typing import Optional
 from pydantic import BaseModel, Field